Cops on overtime are flooding 21 crime hot spots before rookies take over the job at the end of the month, police sources told The Post.

Besides the patrol cops, who are aimed at preventing crime, the NYPD has ordered officers in detective, warrant, gang and narcotics units to focus on solving crimes that have already occurred in those areas.

“It’s to continue to drive crime down in the city,” one source said.

“Operation Impact” began Friday night with approximately 1,200 cops on overtime patrolling three spots in Manhattan, three in The Bronx, eight in Brooklyn, five in Queens and two in Staten Island, sources said.

Patrol borough commanders identified the spots as ones where violent crime, including shootings and robberies, or quality-of-life crimes, such as panhandling and loitering, have been known to occur.

As the patrol cops make new arrests, other officers will be trying to solve crimes that have already happened.

In a memo to all detective borough commanders, Chief of Detectives William Allee told the borough detective captains that they’d be responsible for coordinating Operation Impact.

He said additional detectives would be assigned to spend more time solving old crimes – for example, giving mug shots of wanted suspects to patrol, housing, transit and narcotics cops.

“Warrant squads will obtain a list of persons wanted in each impact area and conduct frequent operations, apprehending outstanding subjects.” Allee said in the Dec. 26 memo obtained by The Post. “Re-interview and energize all CIs [confidential informants], reminding them of possible monetary rewards for information,” he added.

The NYPD is making a five-borough neighborhood push to root out perpetrators in these high-crime areas: (MAP)

Manhattan

* 30th Street to 52nd – Seventh to Eighth Avenues

* 137th Street to 145th St. – Lenox to Seventh Avenues

* 102nd St. to 112th St. – Third to Park avenue

Bronx

* Morrison to Colgate, Bruckner Expressway to 172nd St.

* Cross Bronx Expressway and Jerome Avenue to – University to West Tremont to 177th St.